Birla vishvakrma mahavidyalaya

Prepared by : Kachchhi Jateen M.

Enro no. : 140073106010

Batch: 3D2

Hydrology &water resources engineering

Flood frequency analyses

Guided by : Dr. J. R. Pitroda

Introduction Frequency: “The frequency is the number of time that a given magnitude

flood may occur in a given period.

Knowledge of the magnitude and probable frequency of recurrence of floods is necessary for

1) Economical planning and safe design of : Bridges Dams Levees Culverts, Sewage disposal plants, Other structures located along rivers and streams

2) The effective management of Flood plains Flood defence scheme

3) To predict the possible flood magnitude over a certain time period.

4) To estimate the frequency with which floods of a certain magnitude may occur.

Knowledge of flood frequency is necessary also to flood insurance and flood zoning.

Either overdesign or underdesign of structures involves excessive costs on a long-time basis. The initial cost of a bridge that is designed to pass only a 5-year flood may be small, but the cost of rebuilding it at an average 5-year interval would be large. A bridge at the same site which is built to pass the 100-year flood might be extremely costly. Intermediate design would provide a bridge at that site for the lowest average annual cost.

For the flood frequency analysis use past flood data. The discharge records or past data collected by the Geological Survey and

other Federal, State, and private agencies.

Flood frequency analysis uses historical records of peak flows to produce guidance about the expected behavior of future flooding determination of the design flood. • There are two methods of collecting the flood peak data.

1) Annual-flood series 2) Partial-duration series •An annual flood is defined as the highest momentary peak discharge in a water year. The use of only one flood in each year is the most frequent objection to the use of annual floods.

For the evaluation of flood frequency can be classified into two approaches:

(1) Statistical approach estimates flood quantiles by applying probability models.

(2) The design storm method uses the rainfall-runoff model. • In statistical Method: 1) Probability plotting method 2) Weibull formula 3) Hazen method 4) California method 5) Gumbel’s method

• From this all method Weibull formula is most commonly used.

A flood frequency curve can be constructed by plotting a graph of discharge versus probabilities.

This can easily be accomplished provided you have a data set of annual peak discharge measured over a number of years.

We need to calculate the chronological order of each of the floods. Start by ordering our data according to the magnitude of the flood, from largest to smallest.

Number each of the floods in order, starting with the largest flood as number "1".

The order of the flood is denoted by the letter "m". If you have 100 years of records, you will calculate flood orders for m=1, m=2, m=3,....m=10.

We find the probabilities of flood by this equation, P = m / (n+1)

Where , P= probability m= the number we calculated n=number of years in the record,

probabilities will go on the x axis; and discharge will go on the y axis. title of graph “Flood Frequency Curve.”

Plot corresponding discharges and probabilities.

Draw a best fit line between the data set. The resulting line is the flood frequency curve.

The likelihood of a particular event is calculated from its flood frequency as follows:

F = P × 100 = [ m / (n+1) ] × 100

For example, the chance of a 50-year flood occurring this year is: 1/50 = 0.02 = 0.02 × 100 = 2% chance.

Year Discharge Discharge (descending

order)

M P=M/(N+1)(%)

1950 2000 2684 1 0.019

1951 2100 2100 2 0.039

1952 2684 2000 3 0.059

1953 1251 1500 4 0.078

- - - - -

- - - - -

2000 1500 1251 50 0.98

We also draw graph between Flood recurrence interval and discharge.

This plotting position is in terms of years.

The objective of the frequency analysis is to determine the magnitude of the flood which will be equaled or exceeded once in a specified period of years; this specified period of years is known as the recurrence interval.

“Recurrence intervals" or “Return period " is the time period over which it is likely that a particular magnitude flood will occur.

Based on considerations of probability, several methods have been used to compute the recurrence interval.

For example, if a town has a flood recurrence interval of 100 years, the town on average will flood one time every 100 years.

Calculate the recurrence interval, which is the number of times in your record that a flood of a given magnitude occurred.

Recurrence interval, which is the reciprocal of the annual exceedance probability.

The formula for recurrence interval is. T = 1/P = (n+1)/m

Construct graph on the semi logarithmic paper.

Recurrence interval will go on the x axis; and discharge will go on the y axis. title of graph “Flood Frequency Curve”

Plot corresponding discharges and recurrence intervals.

Draw a best fit line between the data set. The resulting line is the flood frequency curve.

Year Discharge Discharge (descending

order)

M P=M/(N+1) T=1/P

1950 2000 2684 1 0.019 51.0

1951 2100 2100 2 0.039 25.5

1952 2684 2000 3 0.059 17.0

1953 1251 1500 4 0.078 12.75

- - - - - -

- - - - - -

2000 1500 1251 50 0.98 1.0

REFRENSES https://www.google.co.in/?gws_rd=ssl#q=flood+frequency+analysis+methods

http://stream2.cma.gov.cn/pub/comet/HydrologyFlooding/flood/comet/hydro/basic/FloodFrequency/print_version/01-introduction.htm

FLOOD-FREQUENCY ANALYSES by TATE DALRYMPLE manual of hydrology: part 3. flood- flow techniques

TECHNIQUES FOR ESTIMATING FLOOD-FREQUENCY DISCHARGES FOR STREAMS IN IOWA u.s. geological survey water-resources investigations report 00-4233

Thank you…..